In the past, sliding sleeves at various depths and orientations in the wellbore had to be shifted to permit various downhole operations. If the shifting tools were run into the wellbore on rigid tubing, it was usually not too severe a problem to apply a pushing force through the tubing. However, as wellbores became more deviated and with the advent of coiled tubing to decrease the duration of trips into and out of the well, situations arose more often where a force was required to move a sliding sleeve downwardly but could not be provided through the coiled tubing supporting the shifting tool. While, to some degree, the coiled tubing could be used to provide uphole shifting forces when placed in tension, it was more problematic to put the coiled tubing in compression and have any kind of meaningful force applied downhole where the shifting tool engaged the sleeve.
Prior designs which attempted to apply a booster force downhole in coiled tubing applications involved complex mechanisms for anchoring which had the potential for unexpected release due to failure to obtain a solid grip and the inability to provide feedback for alignment so that the surface personnel could know that the booster mechanism had anchored itself in the proper location. Additional repeated shifting using such prior designs resulted in gripping in a similar spot in the tubing with the slips which could gouge or mar the tubing wall, potentially causing downtime. Typical of such designs is U.S. Pat. No. 5,070,941. This patent has a thorough discussion of the prior art pertinent to anchoring devices and piston cylinder combinations.
The apparatus of the present invention represents an improvement over the prior designs in that it presents a simple design which not only provides feedback on whether the downhole operation has been properly accomplished but also, due to its configuration, properly positions the power stroking apparatus for a firm and reliable anchoring which can be easily set and released repeated times while downhole in a reliable manner. It also provides for a simple release because the use of dogs eliminates risk of use of slips that may be difficult to disengage.